Integrated circuits with protected resistors and methods for fabricating the same

ABSTRACT

Methods and apparatus are provided for an integrated circuit with a transistor and a resistor. The method includes depositing a first dielectric layer over the transistor and the resistor, followed by an amorphous silicon layer. The amorphous silicon layer is implanted over the resistor to produce an etch mask, and the amorphous silicon layer and first dielectric layer are removed over the transistor. A contact location on the transistor is then silicided.

TECHNICAL FIELD

The technical field generally relates to integrated circuits and methodsfor fabricating integrated circuits, and more particularly relates tointegrated circuits with resistors and transistors, where the resistorsare protected by an etch mask while the transistors are silicided, andmethods for fabricating the same.

BACKGROUND

Integrated circuits include many electronic components connected invarious ways. A field effect transistor (FET) is a type of electricalcomponent having a source, a drain, a gate, and associated electricalcontacts. In many embodiments, the surface of the transistor issilicided prior to forming an electrical contact as the silicidedsurface is very conductive and improves the electrical connection at thetransistor. As integrated circuits become smaller, the area of thesilicided surface becomes smaller, so the integrity and quality of thesilicided surface becomes more important.

Resistors are required in many electrical circuits for various purposes,e.g. as voltage dividers or as fuse resistors. The resistors aredesigned and built for a particular resistance, and proper operation ofthe electrical circuit depends on the expected target resistance andspecifications for the resistor being fulfilled. Several types ofresistors are common in integrated circuits, such as silicided resistorsthat have a lower resistivity and non-silicided resistors with a higherresistivity. Therefore, non-silicided resistors are typically protectedwhile the contact locations on the transistors are silicided. This caninvolve covering both the transistors and the resistors with variouslayers, which are typically dielectric materials, and then removing thelayers over the transistor for the siliciding process. However,transistors are being placed closer and closer together to developsmaller and smaller integrated circuits, and this creates smaller gapsbetween the gates of adjacent transistors. As the gates are placedcloser together, it is becoming more difficult to remove all of thedielectric material between adjacent gates, especially when the coveringlayers are so thick that they can merge in the space between adjacenttransistor gates. This can result in incomplete removal of thedielectric layer between the gates, which produces dielectric residualson surfaces of the transistor. The dielectric residuals can interferewith the siliciding process, which in turn interferes with theelectrical connection between the transistor and other electricalcomponents. Increasing the etching process to remove the dielectricresiduals results in undesirable, excessive etching on other componentsof the integrated circuit.

Accordingly, it is desirable to develop new systems and methods forprotecting resistors while transistors are silicided. In addition, it isdesirable to develop new integrated circuits with thin protective layersover the resistors. It is also desirable to develop systems and methodsto lower the likelihood of dielectric residuals on transistors prior tosiliciding the transistor contact locations. Furthermore, otherdesirable features and characteristics of the present embodiment willbecome apparent from the subsequent detailed description and theappended claims, taken in conjunction with the accompanying drawings andthis background.

BRIEF SUMMARY

In an exemplary embodiment, a method is provided for producing anintegrated circuit with transistors and resistors. A first dielectriclayer is deposited over the transistor and the resistor, followed by anamorphous silicon layer. The amorphous silicon layer is implanted overthe resistor to produce an etch mask, and the amorphous silicon layerand first dielectric layer are removed over the transistor. A contactlocation on the transistor is then silicided.

In an alternate exemplary embodiment, a method is provided for producingan integrated circuit with transistors and resistors. The resistor isprotected with an etch mask and a first dielectric layer, where thefirst dielectric layer is between the resistor and the etch mask. Theetch mask has a thickness of no more than about 200 angstroms. Thetransistor is exposed, and a contact location on the transistor issilicided.

An integrated circuit is provided in yet another exemplary embodiment.The integrated circuit includes a transistor and a resistor overlying asubstrate. A first dielectric layer is overlying the resistor, and acarbon doped amorphous silicon etch mask is overlying the firstdielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments will hereinafter be described in conjunctionwith the following drawing figures, wherein like numerals denote likeelements, and wherein:

FIGS. 1-9 illustrate in cross sectional views a method of manufacture ofan integrated circuit; and

FIG. 10 illustrates a portion of the integrated circuit in crosssectional view.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the application and uses of the embodimentdescribed. Furthermore, there is no intention to be bound by any theorypresented in the preceding background or the following detaileddescription. The various tasks and process steps described herein may beincorporated into a more comprehensive procedure or process havingadditional steps or functionality. Various steps in the manufacture ofsemiconductor based integrated circuits are well known and so, in theinterest of brevity, many conventional steps will only be mentionedbriefly herein or will be omitted entirely without providing thewell-known process details.

Transistors and resistors are often electrically isolated from eachother by silicon oxide in a shallow trench isolation, and thesedifferent electronic components are processed in different manners.Contact surfaces of transistors are often converted to a highlyconductive metal silicide before making an electrical connection. Ametal is deposited on the contact surface and thermally annealed to formthe metal silicide, but the metal will form a silicide with othermaterials containing silicon as well. Many resistors include silicon,and the formation of a metal silicide changes the designed resistance ofthe resistor. The resistor is protected from the siliciding process witha protective etch mask that also serves as a siliciding mask. A firstdielectric layer is deposited over the transistor and the resistor,followed by an amorphous silicon layer. A photoresist is used to shieldthe amorphous silicon over the transistor, and an etch mask is createdover the resistor by implanting the amorphous silicon with an implantmaterial. The transistor is then exposed by removing the overlyinglayers. The amorphous silicon layer can be essentially completelyremoved without damaging other areas of the integrated circuit, andwithout removing the etch mask, which reduces the propensity fordielectric residuals between adjacent transistor gates. The transistoris then cleaned and a metal silicide is formed at desired contactlocations. The implanted amorphous silicon layer shields the resistorfrom the siliciding process, and the first dielectric layer under theamorphous silicon layer isolates the resistor from the implantedamorphous silicon layer.

FIG. 1 illustrates an exemplary embodiment of an integrated circuit 8having a substrate 10 with a pair of transistors 12 and a resistor 30coupled to the substrate 10. The transistor 12 has a source 14 and adrain 16 in the substrate 10, with a gate 18 extending over thesubstrate 10. As used herein, “overlying” or “over” means “on” (suchthat the gate 18 physically contacts the substrate 10), or “above” (suchthat another material layer may lie in between the gate 18 and thesubstrate 10). A gate insulator 20 is located between the gate 18 andthe substrate 10, and transistor sidewall spacers 22 extend from thesubstrate 10 past the gate insulator 20 to a point near the top of thegate 18. The substrate 10 is a semiconductor, such as crystallinesilicon, but other semiconductors can also be used. A dopant isimplanted into the source 14 and drain 16, where the dopant can beeither a “P” or “N” type doping agent, and the source 14 and drain 16are activated by thermal annealing. The gate 18 can include apolysilicon material, a refractory metal, or other materials. The gateinsulator 20 is a dielectric, and the transistor sidewall spacers 22 arealso a dielectric. For example, the gate insulator 20 may be siliconoxide, and the transistor sidewall spacers 22 may be silicon nitride,but other dielectric materials can also be used. Other embodiments ofthe transistor 12 are also possible. For example, different shapedtransistors 12 can be utilized, such as fin shaped transistors 12.

Different regions on the substrate 10 are isolated from each other byshallow trench isolation 24 (STI). The STI 24 is formed from adielectric material, such as silicon oxide or silicon nitride, but otherdielectric materials can also be used. The embodiment shown in FIG. 1illustrates the resistor 30 separated from the transistors 12 by an STI24, but in other embodiments there is no STI 24 between the resistor 30and the transistor 12. There may be other electronic components presentthat are not illustrated, and in some embodiments many other electroniccomponents are present.

In one embodiment, the resistor 30 is isolated from the substrate 10with a resistor insulator 32, and resistor sidewall spacers 34 extendfrom the substrate 10 past the resistor insulator 32 to a point near thetop of the resistor 30. The resistor 30 can be formed from a variety ofmaterials, such as doped crystalline silicon, doped poly silicon, orother materials. The material of the resistor 30 has a certain naturalresistance to the flow of electricity, and the length, width, andthickness of the resistor 30, combined with the material of the resistor30, determine the resistance. The resistance of the resistor 30 can bechanged by either changing the material of the insulator to a more orless electrically conductive material by additional dopant implants, orby changing the dimensions of the resistor 30. Electrical connections(not shown) are made at or near opposite ends of the resistor 30, socurrent flows through the body of the resistor 30. Other types ofresistors 30 can be used in other embodiments. For example, a portion ofthe substrate 10 can be doped and annealed, so the resistor 10 isembedded in the substrate 10 instead of overlying it. In embodimentswhere the resistor 30 is implanted and annealed, the implantation andannealing are complete at the stage illustrated in FIG. 1.

The resistance of the resistor 30 depends on the material of theresistor 30, at least in part, so a change to the material of theresistor 30 changes the overall resistance. In cases where the resistor30 has a high resistance, such as over about 100 ohms, the addition of ahighly conductive metal silicide to the resistor 30 can significantlychange the overall resistance. Electrical current easily flows throughthe highly conductive metal silicide, and thereby lowers the resistance.The resistance of a resistor 30 of less than about 100 ohms will also beundesirably changed by a metal silicide, but not to the same degree as ahigh resistance resistor 30.

Referring now to FIG. 2, a first dielectric layer 40 is deposited overthe substrate 10, the transistor 12, and the resistor 30. The firstdielectric layer 40 is also deposited over the STI 24, and any othercomponents on the substrate 10. In some embodiments, the firstdielectric layer 40 is silicon dioxide, which can be deposited withchemical vapor deposition, but other types of dielectric can also bedeposited. An amorphous silicon layer 42 is then deposited over thefirst dielectric layer 40, as illustrated in FIG. 3. The amorphoussilicon layer 42 can be deposited using several different techniques,including chemical vapor deposition, plasma enhanced chemical vapordeposition, and hot wire catalytic deposition. Very thin amorphoussilicon layers 42 can be deposited, with total thicknesses of about 100angstroms or less in some embodiments, total thicknesses of about 200angstroms or less in other embodiments, and a wide variety of otherthicknesses. A thin amorphous silicon layer 42 is useful, because theamorphous silicon layer 42 does not “bridge the gap” and connect toitself between adjacent transistor gates 12, or other features thatproduce a tight pitch. This is seen in the “U” shape of the amorphoussilicon layer 42 between the two transistors 12. If the amorphoussilicon layer 42 were to bridge the gap, it would be flat across thespace over and between the two transistor gates 18. However, in someembodiments, the amorphous silicon layer 42 may bridge the gap betweenadjacent gates 12. A thin amorphous silicon layer 42 is also desirablebecause it occupies less space on the integrated circuit 8, and featuresthat occupy less space are desirable as smaller integrated circuits 8are produced.

FIG. 4 illustrates a layer of etch mask photoresist 44 over thetransistor 12. The etch mask photoresist 44 is deposited over thesubstrate 10, including the transistor 12 and resistor 30, thenpatterned and removed from over the resistor 30. The etch maskphotoresist 44 is deposited by spin coating, and patterned using a maskand a source of light or other electromagnetic radiation. A positive ornegative etch mask photoresist 44 can be used. The amorphous siliconlayer 42 is exposed over the resistor 30, but the etch mask photoresist44 protects or shields the amorphous silicon layer 42 over thetransistor.

FIG. 5 illustrates an implant material 46 being implanted into theamorphous silicon layer 42 over the resistor 30. The implant material 46changes the chemical nature of the amorphous silicon and creates an etchmask 48 from the amorphous silicon layer 42. In some embodiments, theimplant material 46 is carbon, and the amorphous silicon layer 42 isconverted to carbon doped amorphous silicon. Implanting the implantmaterial 46 can also further amorphize the amorphous silicon layer 42.However, different implant materials 46, such as boron, can be used inother embodiments. It is relatively easy to control the implant processwhen carbon is used as the implant material 46. The implant angle isvaried, so the relatively vertical portions of the etch mask 48 areimplanted along with the horizontal portions over the resistor 30. Theetch mask 48 is used to protect the resistor 30, and the relativelyvertical side surfaces provide some of that protection. If therelatively vertical side surfaces were not implanted, the etch mask 48could fail at those side surfaces. The implant material 46 does notpenetrate the etch mask photoresist 44, so the amorphous silicon layer42 over the transistor 12 is not implanted. The etch mask 48 has an etchmask thickness 50 that is less than 200 angstroms in some embodiments,and less than 100 angstroms in other embodiments, but other etch maskthicknesses 50 are also possible. The etch mask 48 is created from theamorphous silicon layer 42, so the etch mask thickness 50 is determinedby the thickness of the amorphous silicon layer 42.

The implant material 46 can be implanted into the amorphous silicon byion implantation. The implant energy is chosen such that the underlyingfirst dielectric layer 40 is not penetrated by the implant material 46,so the implant energy depends on the thickness of the amorphous siliconlayer 42. In some embodiments, high doses of implant material 46 areused to ensure complete transformation of the amorphous silicon layer 42into a functional etch mask 48. Various implant materials 46 can beused, but carbon or boron difluoride (BF2) provide good wet etch ratedifferences compared to the unimplanted amorphous silicon layer 42. Awide variety of angles can be used for twist and tilt so the implantmaterial 46 is implanted into vertical side surfaces facing in differentdirections.

After the etch mask 48 has been created from the amorphous silicon layer42, the various materials over the transistor 12 are removed, asillustrated in FIG. 6. The etch mask photoresist 44 can be removed withplasma etching or a wet etchant, such as organic photoresist stripperswith aryl sulfonic acids. The amorphous silicon layer 42 can then beremoved with an ammonia etch. Ammonia has a slow etch rate for siliconoxide, so a silicon oxide first dielectric layer 40 protects thetransistor 12 from the ammonia etch. This allows for essentiallycomplete removal of the amorphous silicon layer 42, which greatlydecreases the chances of any dielectric residuals between the transistorgates 18. Ammonia also has a slow etch rate for the etch mask 48 overthe resistor 30, so the etch mask 48 remains in place. Once theamorphous silicon layer 42 is removed, the first dielectric layer 40 isremoved using any of several possible methods. For example, if the firstdielectric layer 40 is silicon oxide, it can be removed with ahydrofluoric acid liquid etchant, which may be diluted with water orbuffered and diluted with ammonium fluoride. The STI 24 may be recessedsomewhat from the surface of the substrate 10, and a portion of thefirst dielectric layer 40 is left over the STI 24 to form a flushsurface with the substrate 10. The first dielectric layer 40 over theSTI 24 helps to isolate the areas of the substrate 10 on opposite sidesof the STI 24, which helps prevents shorts over the STI 24.

The siliciding process will form metal silicides from any exposedsurface with free silicon. Therefore, the first dielectric layer 40 isremoved from all free silicon surfaces that are to be silicided,including a desired contact location 52. For example, silicon in thesource 14, drain 16, and gate 18 are exposed when the first dielectriclayer 40 is removed. The transistor sidewall spacers 22 are made of adielectric material, such as silicon nitride, so no metal silicide willform on the transistor sidewall spacers 22. The desired contact location52 is a location where an electrical contact will be formed. The etchmask 48 protects the resistor 30 from the siliciding process, so theetch mask 48 also functions as a siliciding mask. The etch mask 48produced over the resistor 30 can also be produced over any othersurfaces where a metal silicide is not wanted, so the etch mask 48 canprotect electrical components other than the resistor 30 from thesiliciding process. The process described above for forming the etchmask 48 is also used to protect other electrical components or surfacesfrom siliciding.

Reference is now made to FIG. 7. After the first dielectric layer 40 isremoved from over the transistor 12, the surface is prepared forsiliciding. The siliciding process includes several steps, includingcleaning, depositing a layer of metal 51, and then annealing the contactlocation 52 to form the metal silicide. The exposed substrate 10,transistor 12, and etch mask 48, are cleaned with an organic solutionfollowed by a dilute hydrofluoric acid solution, and then rinsed withdeionized water and dried. Other cleaning embodiments are also possible,such as dipping the integrated circuit 8 in dilute hydrofluoric acid,followed by a drying step. The integrated circuit 8 is then optionallyplaced in a metal deposition chamber, and further cleaned with sputteredargon ions. A thin layer of metal 51 is then deposited on the exposedsurfaces, such as by sputtering or chemical vapor deposition. Severaldifferent metals 51 can be deposited for the siliciding process, such asnickel (Ni), titanium (Ti), or cobalt (Co). After the layer of metal 51is deposited, the metal silicides are formed by thermally annealing thecontact locations 52. Annealing temperatures of about 100° C. to about700° C. are used in some embodiments, and the annealing temperature isselected based on the metal used. In an exemplary embodiment usingnickel (Ni), a two step anneal is used. The first annealing step usestemperatures from about 240° C. to about 320° C. for about 10 to about40 seconds, and this step defines the diffusion of the Ni into thesilicon material, such as polysilicon. A second annealing step is usedto convert the silicon and nickel to a stable nickel-silicide phase withspecific resistance values. The second annealing step uses temperaturesfrom about 400° C. to about 500° C. for about 20 to about 40 seconds.However, in some embodiments ultra-short laser anneals are used.

Referring now to FIG. 8, with continuing reference to FIG. 7, the metal51 reacts with silicon to form a metal silicide 53, but the metal 51does not react with the dielectrics or other materials. Wet etching isthen used to remove the layer of metal 51 from areas where a metalsilicide 53 was not formed and also any remaining metal that did notdiffuse into the silicon material, such as the polysilicon. For example,nickel can be selectively etched with a mixture of nitric acid, aceticacid, and sulfuric acid. Cobalt can be selectively etched with a mixtureof hydrochloric acid and peroxide. Titanium can be selectively etchedwith a mixture of ammonia and peroxide. Many other etchants can be usedin various embodiments.

Once the contact locations 52 are silicided, an interlayer dielectric 54is deposited over the exposed surfaces, including the substrate 10, thecontact locations 52, the transistors 12, and the etch mask 48, asillustrated in FIG. 9. The interlayer dielectric 54 is silicon oxide insome embodiments, but other dielectric materials can be used in otherembodiments. Silicon oxide can be deposited by chemical vapordeposition.

A contact 56 is formed through the interlayer dielectric 54 so that itextends through the interlayer dielectric 54 and electrically connectsto the metal silicide 53 at the contact location 52. The contact 56 canbe formed by depositing a hard mask 58, such as titanium nitride orother materials, on top of the interlayer dielectric 54. Atomic layerdeposition can be used to deposit titanium nitride for the hard mask 58.A contact photoresist layer 60 is then deposited over the hard mask 58,such as by spin coating. The contact photoresist layer 60 is patternedand a via is etched through the hard mask 58 and the interlayerdielectric 54 to the contact location 52. The contact 56 is thendeposited within the via. In some embodiments, the contact 56 is formedby sequentially depositing an adhesion layer, a barrier layer, and aplug. In an exemplary embodiment, an adhesion layer of titanium isdeposited by atomic layer deposition, a barrier layer of titaniumnitride is deposited by atomic layer deposition, and a plug of titaniumis deposited by chemical vapor deposition, but other embodiments arealso possible. The contact photoresist layer 60 and hard mask 58 arethen removed, as well as the portion of the contact 56 extendingtherethrough, as illustrated in FIG. 10. The contact 56 at the uppersurface of the interlayer dielectric 54 is then available for electricalconnections to other electronic components for the integrated circuit 8.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theapplication in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing one or more embodiments, it being understood that variouschanges may be made in the function and arrangement of elementsdescribed in an exemplary embodiment without departing from the scope,as set forth in the appended claims.

What is claimed is:
 1. A method of producing an integrated circuitcomprising: depositing a first dielectric layer over a resistor and atransistor of the integrated circuit; depositing an amorphous siliconlayer over the first dielectric layer; implanting the amorphous siliconlayer over the resistor to create an etch mask; removing the amorphoussilicon layer and the first dielectric layer overlying the transistorwhile leaving the etch mask over the resistor; and siliciding a contactlocation on the transistor.
 2. The method of claim 1 wherein implantingthe amorphous silicon layer further comprises implanting carbon into theamorphous silicon layer to produce the etch mask comprising carbon dopedamorphous silicon.
 3. The method of claim 1 wherein siliciding thecontact location further comprises depositing a metal on the contactlocation and thermally annealing the contact location.
 4. The method ofclaim 3 wherein depositing the metal further comprises depositing themetal comprising nickel on the contact location.
 5. The method of claim1 wherein depositing the first dielectric layer over the resistor andthe transistor further comprises depositing silicon oxide over theresistor and the transistor.
 6. The method of claim 1 wherein implantingthe amorphous silicon layer to create the etch mask further comprises:depositing an etch mask photoresist over the transistor and theresistor; patterning the etch mask photoresist; and removing the etchmask photoresist over the resistor while leaving the etch maskphotoresist over the transistor.
 7. The method of claim 1 furthercomprising forming a contact to the contact location after silicidingthe contact location.
 8. A method of producing an integrated circuitcomprising: protecting a resistor of the integrated circuit with a firstdielectric layer and an etch mask, wherein the first dielectric layer ispositioned between the resistor and the etch mask, wherein the etch maskhas an etch mask thickness of no more than about 200 angstroms, andwherein creating the etch mask comprises implanting an implant materialinto an amorphous silicon layer; exposing a transistor of the integratedcircuit, wherein exposing the transistor comprises removing theamorphous silicon layer from over the transistor while the etch maskremains over the resistor; and siliciding a contact location on thetransistor.
 9. The method of claim 8 wherein protecting the resistorfurther comprises: depositing the first dielectric layer over theresistor; and depositing the amorphous silicon layer over the firstdielectric layer.
 10. The method of claim 9 wherein implanting theimplant material further comprises implanting carbon into the amorphoussilicon layer to produce the etch mask comprising carbon doped amorphoussilicon.
 11. The method of claim 8 wherein exposing the transistorfurther comprises removing an etch mask photoresist, and the firstdielectric layer from over the transistor.
 12. The method of claim 8wherein siliciding the contact location further comprises depositing ametal over the contact location, and annealing the contact location. 13.The method of claim 12 wherein depositing the metal over the contactlocation further comprises depositing the metal comprising nickel overthe contact location.
 14. The method of claim 8 wherein protecting theresistor further comprises depositing the first dielectric layercomprising silicon oxide over the resistor.
 15. The method of claim 8further comprising forming a contact to the contact location aftersiliciding the contact location.